a) Field of the Invention:
The present invention relates to a headlamp unit for motor vehicles, and more particularly to a so-called "slant type" headlamp unit of which the front lens is slanted with respect to the optical axis of the reflector.
b) Description of the Related Art:
One of the requirements imposed on the headlamps for motor vehicles is to illuminate over a lane or road without dazzling the driver of a coming car on the opposite lane or in the opposite direction. To meet this requirement, various types of headlamp units have so far been proposed. A typical one of such conventional headlamp units comprises a reflector having an inner reflecting surface like a paraboloid of revolution, a lamp bulb as light source disposed near the focus of the reflector, and a front lens covering the front opening of the reflector and disposed nearly perpendicularly to the optical axis. The headlamp unit of this type is designed so that the rays of light emitted from the light source and incident upon the inner reflecting surface of the reflector is reflected frontwardly as fluxes of light generally parallel to the optical axis and diverged horizontally to the right and left, thereby providing a desired luminous intensity distribution pattern.
These days, the style of car bodies have been streamlined in order to minimize the large air resistance against a running car and also for meeting the aesthetic requirements. In these circumstances, headlamp units of a new type (slant type) have been proposed which are wholly flush with the body line of the car. In the headlamp units of this type, the front lens is inavoidably slanted with respect to the optical axis. If the basic structure of the above-mentioned headlamp units is applied as it is to the slant-type headlamp unit, the opposite ends of the luminous intensity distribution pattern droop somehow, so it is impossible to illuminate a sufficient area of the lane or road surface for the driver to recognize the traffic signs installed by the side of the lane or a person going to cross the lane.
To avoid such slight droop of the opposite ends of the luminous intensity distribution pattern, a headlamp unit has been proposed which has formed at a part of the inner reflecting surface of the reflector which has the shape of a paraboloid of revolution, a light-diverging reflecting area which diverges the rays of light incident from the light source. One example of such proposed headlamp units is disclosed in the Japanese Unexamined Utility Model Publication No. 63-12101 (laid open on Jan. 26, 1988).
FIG. 1 is a schematic diagram of such headlamp unit, and FIG. 2 is a schematic front view of a reflector having the light-diverging reflecting area. As seen in these Figures, the axis X--X indicates the horizontal plane in which the optical axis of the reflector 10 lies, the axis Y--Y indicates the vertical plane in which the optical axis of the reflector 10 lies, and the axis Z--Z indicates the optical axis of the reflector 10. As shown in FIG. 2, the reflector 10 has formed at the center thereof a central opening 11 for fixing a lamp bulb 12 or light source, and it consists of a main reflecting area 20 made of a part of a paraboloid of revolution (the main reflecting areas formed in positions nearly symmetrical with respect to the vertical plane in which the optical axis lies are indicated with reference numerals 20a and 20b, respectively), and a light-diverging reflecting area 22 located above the horizontal plane in which the optical axis lies and formed in a position surrounding the central opening 11 (the light-diverging reflecting areas formed in positions generally symmetrical with respect to the vertical plane in which the optical axis lies). The lamp bulb 12 is of a type having two filaments: main filament 12a and sub filament 12b. Usually, the lamp bulb of this type has the center of the main filament 12a positioned nearly at the focus F of the paraboloid of revolution and the center of the sub filament 12 b positioned at a position displaced toward the front lens. Namely, the rays of light emitted from the sub filament 12b and reflected at the main reflecting area 20 formed by a part of a paraboloid of revolution are not substantially parallel with the optical axis but as somehow convergent toward the optical axis.
The diverging reflecting area 22 consists of multiple light-diverging reflecting elements formed as directed longitudinally. The horizontal section of each of the light-diverging reflecting elements takes the form of a concave part of a circle while the vertical section takes the form of a parabola having a focus at the point F. Thus, theoretically, the rays of light incident upon the light-diverging reflecting area 22 from the focus of the reflector are diverged in the horizontal plane while being reflected in directions nearly parallel with the optical axis in the vertical plane. Actually, the rays of light emitted from the sub filament 12b at a position off the focus F and incident upon the light-diverging reflecting area 22 are diverged in the horizontal plane and travel as somehow converged toward the optical axis in the vertical plane.
A front lens 13 disposed covering the front opening of the reflector 10 is shown as enlarged in scale in the schematic front view in FIG. 3. There is shown in FIG. 4 a illumination pattern formed with the rays of light emitted from the sub filament 12b, refracted through the front lens 13 and projected onto a screen located 10 meters before the headlamp unit. The front lens 13 is disposed as slanted with respect to the horizontal plane in which the optical axis Z--Z lies while the center O thereof is kept coincident with the optical axis. As shown in FIG. 3, the disposition of the front lens 13 is defined by the axis x--x passing by the center O of the front lens 12 and perpendicular to the optical axis and the axis y--y passing by the center O and slanted with respect to the optical axis. The front lens 13 consists of a plurality of prism areas A, B, C, D, E, F, G and H made of multiple cylindrical prism elements (not shown), respectively. The bottom line of each of prism elements forming the prism areas A, B, C, D, F, G and H is parallel with the axis y--y, while only the bottom line (indicated with one dot dash line) of the prism element forming the prism area E is slanted an angle .theta. with respect to the axis y--y. As shown, the prism areas A and B are formed as elongated horizontally at the upper left and right portions, respectively, of the front lens 13. The prism area F is formed at the central portion in which the optical axis lies. The prism area C is formed as elongated horizontally below the prism area A, and the trapezoidal prism area E is formed below the prism area C. The prism area D is formed as elongated horizontally below the prism area B, and above and below the axis x--x. These prism areas are contributed to the formation of an illumination pattern by the sub filament 12b when energized. The prism areas G and H shown in FIG. 3 are contributed to the formation of an illumination pattern by the main filament when energized, the prism area G providing an illuminated zone at the center of the luminous intensity distribution pattern where the luminance is very high while the prism area H forms a horizontally elongated illuminated zone extending to the right and left from the center of the luminous intensity distribution pattern and in which the luminance is relatively low. However, since these prism ares G and H have nothing to do directly with the subject of the present invention, they will not be further described hereinbelow.
The rays of light reflected at the main reflecting surface areas 20a and 20b formed by a part of a paraboloid of revolution are incident upon any of the prism areas A, B, C, D, E and F, while the rays of light diverged at the light-diverging reflecting areas 22a and 22b are incident upon any of the prism areas D, E and F. The rays of light refracted through these prism areas are projected to form the illumination patterns on a screen as will be described below.
The rays of light reflected at the main reflecting area 20b and then incident upon the prism area A, those reflected at the main reflecting area 20a and incident upon the prism area B, and those reflected at the main reflector areas 20a and 20b and incident upon the prism area F form horizontally elongated illumination patterns (A), (B) and (F), respectively, below the horizontal plane and where the luminance is relatively low. The rays of light reflected at the main reflecting area 20b and then incident upon the prism area C form an illumination pattern (C) extending from the center toward the lower left and in which the luminance is relatively high. The rays of light reflected at the main reflecting area 20a and then incident upon the prism area D form an elongated illumination pattern (D) below the horizontal plane and where the luminance is relatively high. Further, the rays of light reflected at the main reflecting area 20b and then incident upon the prism area E form an illumination pattern (E) extending from the center toward the upper left and in which the luminance is relatively high.
On the other hand, the rays of light diverged at the light-diverging reflecting areas 22a and 22b are incident upon elongated horizontal zones D0, E0 and F0 (indicated as enclosed with dash line in FIG. 3) located above the horizontal plane X--X in the prism areas D, E and F. The rays of light incident upon the horizontal areas DO and FO in the prism areas D and F form horizontally elongated illumination patterns (DO) and (FO) below the horizontal plane and in which the luminance is relatively low. In the prism area E, the bottom line of each of the cylindrical prism elements is slanted an angle .theta. with respect to the axis y--y, so the rays of light diverged at the light-diverging reflecting areas 22a and 22b and then incident upon the elongated horizontal area EO in the prism area E are travel in the direction of arrows j-k. Therefore, a horizontally elongated illumination pattern below the horizontal plane, such as formed by the horizontal areas DO and FO, cannot be formed but an illumination pattern (EO) is formed as derived from the shift of a horizontally elongated pattern in the direction of arrow P and in an opposite direction Q to the direction P.
Illumination patterns formed by the prism areas are superposed on one another to form an actual luminous intensity distribution pattern, thereby overcoming the drawback of the conventional technique that the luminous intensity distribution pattern bends with the opposite ends thereof drooping somehow. As having been described in the above, however, since the rays of light diverged at the light-diverging reflecting areas 22a and 22b are further diverged by the prism elements forming the horizontal area EO in a direction perpendicular to a bottom line slanted an angle .theta. with respect to the axis y--y, that is, in the direction of arrows j-k, the illumination pattern (EO) projected on the screen protrudes very much above the profiles of the horizontally elongated illumination patterns (A), (B), (D), (D), (DO) and (FO), which is another drawback of the conventional technique. Actually in such luminous intensity distribution pattern, the horizontal cut line is indefinite and low in luminance, and in the pattern projected on the road surface, the luminance of the illumination pattern (EO) is relatively low, but a part of the pattern extends to the opposite traffic lane.